The invention refers to a process for extraction or recovery of acids, in particular hydrofluoric acid, hydrochloric acid and nitric acid, from solutions of these acids containing metal, by pyrohydrolytic treatment, separation of the metals and subsequent absorption and/or condensation of the thus formed acid gases in an aqueous absorption solution.
Solutions of hydrofluoric, hydrochloric or nitric acid containing metal are produced in the metal industry, for example, in the form of mixed acid waste pickle liquor from the surface treatment of standard steel grades, stainless steels, special alloys and special metals. These solutions contain free acids, such as hydrofluoric acid, hydrochloric acid and nitric acid, as well as dissolved metals, such as iron, chrome and nickel, in the form of fluorides, nitrides and chlorides and they have to be renewed as from a metal concentration of approximately 35-120 g/l, depending on their application. A number of methods have already been developed to recover the hydrofluoric acid, hydrochloric acid and nitric acid from the used solutions for recycling back to the pickling process and, to a large extent, these methods also avoid the problems of disposal which arise due to environmental regulations and the ever increasing disposal costs. The methods already known for recovery of hydrofluoric acid, hydrochloric acid and nitric acid are, for example, solvent extraction, dialysis and the use of ion exchangers.
In AT-PS 395.312, the author also suggested releasing these acids from solutions containing metals by means of the pyrohydrolytic stage of the spray roasting and then either absorbing and/or condensing the gases produced. It is preferred to conduct absorption and/or condensation in two columns, in which process the amount of hydrofluoric acid, hydrochloric acid or nitric acid extracted can be improved by controlling the column temperature.
Low temperatures (&lt;50.degree. C.) are particularly important for recovery of nitric acid from NO.sub.x. In this process, however, the acid regenerated is diluted because of the water condensation from the exhaust gas, so large amounts of fresh acid have to be added in order to reach the concentration required for use in the pickling process. The continuous increase in volume also creates the need to discard a part of the waste pickle liquor or the regenerate or to neutralize it. Even if the regenerated acid is split into a portion rich in hydrofluors and a more dilute acid portion, there is less need to acidify further, however the dilute acid is not always required and part of it has to be discarded or neutralized.
In addition, the amount of nitric acid extracted drops automatically if the hydrofluor concentration in the regenerate is increased by splitting into acids with a high concentration of hydrofluors and a low concentration of hydrofluors, although the concentrated, regenerated acid only contains the condensable HNO.sub.3 portion and has to be further acidified by adding nitric acid, which again causes dilution.
The energy consumption for this has also been high to date because, on the one hand, the waste pickle liquor has to be evaporated during spray roasting in order to be regenerated and on the other hand, the flow of exhaust gas after the Venturi washer has to be condensed again.